1. Field of the Invention
The present invention relates to apparatuses for measuring a distortion position of an optical fiber, and more particularly, to apparatuses for measuring a distortion position of an optical fiber using an optical frequency shifter capable of forming a Brillouin optical time domain analyzer having Brillouin amplification characteristics using a light source.
2. Background Art
An example of the structure of a conventional apparatus for measuring a distortion position of an optical fiber will be explained with reference to FIG. 5. In FIG. 5, light sources 1A, 1B, a display 7, an optical switch 9A, an optical coupler 9B, and an optical fiber 10, are provided. In FIG. 5, the light source 1A generates continuous light 21 and the light source 1B generates continuous light 22. The continuous light 22 from the light source 1B is converted to an optical pulse 23 by the optical switch 9A. The display 7 has the same structure as a conventional display used in a conventional optical pulse tester. The optical fiber 10 is to be measured. The circuit shown in FIG. 5 forms a Brillouin optical time domain analyzer (hereafter referred to as BOTDA). The BOTDA is disclosed in Japanese Patent Application Laid-Open Publication No. 2-6725.
In FIG. 5, the continuous light 21 from the light source 1A is injected into a far-end 10A of the optical fiber 10. The optical pulse 23 from the optical switch 9A is injected into a near-end 10B of the optical fiber 10 via the optical coupler 9B. The optical pulse 23 progresses toward the far-end 10A in the optical fiber 10 while continuously colliding with the continuous light 21 injected at the far-end 10A in the optical fiber 10. When the frequency difference between the frequency of the continuous light 21 from the light source 1A and that of the continuous light 22 from the light source 1B is equal to Brillouin shift frequency, the continuous light 21 injected at the far-end 10A in the optical fiber 10 progresses toward the near-end 10B while being Brillouin amplified, and thereby the loss characteristics of the optical fiber 10 are included in the continuous light 21 as information. The continuous light 21 from the near-end 10B is supplied to the display 7 via the optical coupler 9B. The loss characteristics of the optical fiber 10 is displayed on the display 7.
In the above conventional apparatus for measuring the distortion position of the optical fiber, since the continuous light 21 and the optical pulse 23 are injected, respectively, in the far-end 10A and the near-end 10B of the optical fiber 10, both of the light sources 1A and 2B are needed. The frequency stability of the light sources 1A and 1B directly influences measurement accuracy of the distortion of the optical fiber 10. Because the Brillouin shift frequency is approximately 500 MHz when the distortion of the optical fiber 10 is 1%, the stability of the relative frequency between the continuous light 21 from the light source 1A and the continuous light 22 from the light source 1B must be less than 5 MHz. However, in the conventional apparatus for measuring the distortion position of the optical fiber, it is not possible for the above stability to be less than 5 MHz.